1. Field of the Invention
The present invention relates to methods and compositions for treating subterranean zones in formations penetrated by well bores utilizing environmentally safe polymer breakers.
2. Description of the Prior Art
Treating fluids that contain polymer breakers are used in a variety of operations and treatments in oil and gas wells. The treatments include well completions and production stimulation treatments. An example of a well completion treatment which utilizes a polymer breaker in a high viscosity fluid is known in the art as gravel packing. In gravel packing treatments, solid gravel particles such as sand are carried by way of the well bore to a subterranean zone in which a gravel pack is to be placed by a viscous gelled carrier fluid. That is, particulate solids (referred to in the art as gravel) are suspended in the high viscosity carrier fluid at the surface and carried to the subterranean zone in which the gravel pack is to be placed. Once the gravel is placed in the zone, the viscous carrier fluid is broken (the viscosity is reduced) and recovered (returned to the surface) by including a delayed polymer breaker, i.e., a viscosity reducing agent, in the carrier fluid. The gravel pack produced functions as a filter to separate formation solids from produced fluids while permitted the produced fluids to flow into and through the well bore.
In open hole gravel packing procedures, a non-viscous carrier fluid can be used that includes a polymer breaker which breaks down drill-in fluid filter cake left on the walls of the open hole well bore from the well bore drilling operation. The carrier fluid for open hole gravel packing can also be viscosified. In that case, the delayed breaker in the carrier fluid breaks the carrier fluid and the filter cake so that the carrier fluid and the filter cake can be removed from the subterranean zone.
An example of a production stimulation treatment utilizing a polymer breaker in a viscous gelled fluid is hydraulic fracturing. That is, a viscous fluid, referred to in the art as a fracturing fluid, is pumped through the well bore into a subterranean zone to be stimulated at a rate and pressure such that fractures are formed and extended into the subterranean zone. The fracturing fluid also carries particulate solids, referred to in the art as proppant particles, into the fractures. The proppant particles are suspended in the viscous fracturing fluid so that the proppant particles are carried into the fractures. The proppant particles function to prevent the fractures from closing whereby conductive channels are formed through which produced fluids can flow to the well bore.
After the viscous fracturing fluid has been pumped into a subterranean zone and fracturing of the zone has taken place, the proppant particles are deposited and the fracturing fluid is removed from the zone to allow produced hydrocarbons to flow through the propped fractures. Generally, the deposition of the proppant particles and the removal of the viscous fracturing fluid are accomplished by converting the fracturing fluid to a low viscosity fluid. This conversion of the fracturing fluid is brought about by including a delayed polymer breaker in the fracturing fluid prior to pumping it into the subterranean zone.
Sodium perborate tetrahydrate which is environmentally safe has been utilized heretofore for breaking viscous fluids. However, in gravel packing treatments, fracturing treatments and other treatments wherein fresh water and salt water based fluids are utilized, sodium perborate tetrahydrate often has limited solubility in the fluids. Thus, there is a need for environmentally safe delayed polymer breakers containing sodium perborate tetrahydrate which are more soluble in fresh water and salt water.